Fluid pressure one-way jar



Dec. 7, 1965 D. D. WEBB FLUID PRESSURE ONE-WAY JAR 3 Sheets-Sheet l Filed Dec. 10, 1963 INVENTOR DERREL D. wEBs ATTORNEY Dec. 7, 1965 D. D. WEBB FLUID PRESSURE ONE-WAY JAR 3 Sheets-Sheet 2 Filed Dec. 10, 1963 INVENTOR DERREL D. WEBB ATTORNEY Dec. 7, 1965 D. D. WEBB FLUID PRESSURE ONE-WAY JAR 3 Sheets-Sheet 5 Filed Dec. 10, 1963 INVENTOR DERREL D. WEBB ATTORNEY Unted States Patent O 35221326 FLUH) PRESURE ONE-WAY EAR Derrel D. Webi), Houston, Tex., assignor to Houston Engineers, Inc., Houston, Ten., a corporation of Texas Filed Dec. 10, 1963, Ser. No. 329,-% 4 Claims. (Ci. 175-297) This invention relates to well tools primarily for use in relatively deep wells and more particularly to a tool which may be lowered into a well and utilized to impart an upward Shock or impact to a pipe string or other tool jammed in the well in order to free the same and permit recovery and is an improvement on the tool disclosed in my prior Patent No. 2,987,l29, patented June 6, 1951.

I-leretofore various types of impact tools or jars have been proposed and utilized to free tools or other devices jammed in a well and permit recovery of the same and certain of these prior art devices have incorporated means for compressing a fluid within the device and suddenly releasing such pressure to permit engagement of impact surfaces thereby providing an upward shock or impact which is transmitted to the tool or pipe string jammed in the well to free the same. These prior ait devices utilizing fluid pressure have been relatively complex and have also been designed in such a manner that the interier of the device is substantially completely filled with mechanism thereby preventing the provision of an aXial passage through which a wire line may be passed in order to carry on Operations in the well below the jar. The design of these prior art tools has also necessitated a relatively large outside diameter which limits the use of the same to wells having a relatively large bore and, therefore, a jar of relatively small outer diameter and providing a relatively large central axial passage which may be utilized for various purposes represents a substantial step forward in the art.

Prior art well tools of the type under consideration have utilized some form of valve means to provide for creating a fluid pressure and thereafter suddenly releasing the same or have utilized longitudinal grooves in the interier wall of the cylinder which cooperate with packing means on the piston to suddenly release the fluid pressure when the packing means on the piston reaches the grooves in the cylinder wall. The formation of longitudinal grooves in a cylinder wall represents a relatively costly machining operation and furthermore, this structure tends to increase the wear on the packing means provided on the piston and consequently an improvement in this area of such tools also represents a substantial step forward in the art.

It is accordingly an object of the invention to provide a fluid pressure one-way jar type well tool which may be lowered into a well and coupled to a pipe or tool jammed therein in relatively proximity to the point where the tool or pipe is jammed to permit the application of an upward shock or impact substantially directly thereto without loss of impact force due to absorption in a relatively long pipe or drill string.

A further object of the invention is the provision of a fluid pressure one-way jar type well tool which may be attached to a pipe or drill string and to a tool or pipe to be recovered from a well without necessitating any modification whatsoever of the pipe or drill string or conventional well tools and in which the tool of this invention is completely self-contained thereby requiring no auxiliary equipment.

A still further object of the invention is the provision of a fluid pressure one-way jar type well tool incorporating a cylinder and a piston movable in such cylinder and in which initial movement of the piston operates to compress a uid in the cylinder until upon reaching a predetermined point in the travel of the piston, the pressure is suddenly reduced to permit relatively rapid movement of the piston in the cylinder.

3221326 Patented Dec. 7, 1965 Another object of the invention is the provision of a fluid pressure one-way jar type well tool incorporating a cylinder and a piston movable in such cylinder and in which initial movement of the piston operates to compress a fluid in the cylinder until upon reaching a predetermined point in the travel of the piston, the pressure is suddenly reduced to permit relatively rapid movement of the piston in the cylinder and in which means is provided to transmit torque through the outer stationary parts of the tool thereby relieving the piston and associated internal parts from torque stresses.

A further object of the invention is the provision of a fluid pressure one-way jar type well tool utilizing a piston and cylinder which is operable to initially build up a fluid pressure and suddenly release the same, there being a relatively large centrally disposed aXial passage extending through the entire tool to permit passage of a wire line or other Operating mechanism therethrough for any desired purpose.

A still further object of the invention is the provision of a fluid pressure one-way jar type well tool utilizing a piston and cylinder which is operable to initially build up a fluid pressure and suddenly release the same, there being a portion of reduced diameter in the cylinder cooperating with packing means on the piston to provide for building up a fluid pressure and there being a portion of enlarged diameter in the cylinder terminating in a tapered portion merging into the portion of reduced diameter whereby upon the packing means on the piston reaching the portion of enlarged diameter fluid pressure will be suddenly released and the tapered portion provides for smooth sliding action of the packing means on the piston between the portion of enlarged diameter and the portion of reduced diameter.

Another object of the invention is the provision of a fluid pressure one-way jar type well tool having a minimum outer diameter and provided with a relatively large, centrally disposed aXial passage extending entirely through the tool.

Further objects and advantages of the invention will be apparent from the following description taken in conjunction with the accomparying drawings wherein:

FIG. 1 is a longitudinal sectional View of the upper portion of a tool constructed in accordance with this invention and showing the means for transrnitting torque from one part of the tool to another part, as well as the shoulder which provides a downwardly facing impact surface;

FIG. 2 is a longitudinal sectional view of an intermediate portion of the tool and showing the hammer means having an upwardly facing striking surface thereon;

FIG. 3 is a longitudinal sectional view showing the major portion of the lower end of the tool, but omitting the means for securing the tool to a well string or other tool to be removed from the well;

FIG. 4 is a fragmentary lorgitudinal sectional view showing the position of a piston ring on the piston at the point where fluid pressure is suddenly released;

FIG. 5 is fragmentary longitudinal sectional view similar to FIG. 4, but showing the position of the piston ring on the piston during upward movement of the piston to develop fluid pressure within the tool;

FIG. 6 is is a sectional view taken substantially on the line 6-6 of FIG. 1 and showing the torque transmitting means between the inner and outer parts of the tool;

FIG. 7 is a sectional view taken substantially on the line 7-7 of FIG. 3; and

FIG. 8 is a sectional view taken substantially on the line 8-8 of FIG. 3 and further showing the structure of the piston, together with the by-pass grooves therein.

With continued reference to the drawings, there is shown a fluid pressure one-way jar comprisng an upper elongated tubular member providing a torque barrel, a cylinder 11 removably secured to the lower end of the barrel 10 by screw threaded fastening means or the like 12 and an elongated sleeve 13 removably secured to the lower end of the cylinder 11 by screw threaded fastening means or the like 14. The sleeve 13 may be provided at the lower end thereof with means, not shown, for coupling the sleeve to a tool or pipe string which has become lodged in the well and is to be removed therefrom by the tool of this invention. An annular shoulder 15 is provided in the upper end of the barrel 10 and the shoulder 15 serves to provide a downwardly facing impact surface 16, the purpose of which will be later described.

A hollow torque mandrel 17 is slidably received in the barrel 10 and the mandrel 17 is provided at the upper end thereof with conventional screw threaded means 18 for attaching the same to a drill string. Means is provided to prevent rotation of the mandrel 17 with respect to the barrel 10, but to permit sliding movement therebetween and such means may take the form of a plurality of elongated pins 19 which, as best shown in FIGS. 1 and 6, are received in circumferentially spaced semicircular recesses 20 in the inner surface of the barrel 10 and the pins 19 are held in position abutting the upper end 21 of the shoulder 15 by a nut 22 threadedly engaging the upperend of the barrel 10 at 23. Longitudinally extending semi-circular grooves 24 are provided in the exterior surface of the mandrel 17 and the pins 19 are slidably received in the grooves 24 and serve to permit sliding movement between the mandrel 17 and barrel 10 but prevent relative rotation therebetween. In this manner, torque` may be transmitted from the mandrel 17 to the barrel 10.

A coupling sleeve 25 is provided with an upper screw threaded portion 26 which threadedly engages the lower end of the mandrel 17 at 27 and the sleeve 25 is provided with a lower portion 28 having screw threaded means 29 threadedly engaging the upper end of a tubular piston rod 30. The coupling sleeve 25 may be provided with an inwardly projecting annular flange 31 which abuts the lower end of the mandrel 17 and the upper end of the piston rod 30. The coupling sleeve 25 also serves to provide an enlarged hammer portion having an upwardly facing striking surface 32 on the upper end thereof which cooperates with the downwardly facing impact surface 16 on the shoulder 15 in the barrel 10 in a manner to be later described.

The tubular piston rod is slidably received within the cylinder 11 and suitable packing means 33 may be provided adjacent the upper end of the cylinder 11 and engaging the outer, sur-face of the piston rod 30 in order to provide a fluid-tight seal therebetween. As best shown in FIG. 3, the hollow piston rod 30 a'lso extends into and is slidably received in the elongated sleeve 13 and suitable packing means 34 may be provided at the upper end of the sleeve 13 engaging the outer surface of the piston rod 30 to provide a fluid-tight seal therebetween. The annular space between the piston rod 30 and the cylinder 11 which lies between the upper packing means 33 and the lower packing means 34 serves to provide a fluid receiving chamber which may be filled With a suitable hydraulic fluid through an opening 35 in the wall of the cylinder 11 'which communicates with the interior thereof through a passage 36 and the opening 35 may be closed with a suitable screw threaded plug 37. As best shown in FIGS. 2 and 3, the cylinder 11 is provided on the interior thereof With a portion 38 of enlarged diameter and such portion extends from a point adjacent the upper end of the cylinder 11 to a point intermediate the length thereof and terminates in an inwardly tapering wall 39 merging into a portion 40 of reduced diameter which extends to the lower end of the cylinder 11.

The piston rod 30 is provided with a piston disposed within the cylinder 11 and such piston may well comprise an annular flange 41 on the piston rod 30 and as clearly shown in FIG. 3, the outer diameter of the flange 41 is less than the inner diameter of the reduced portion 40 of the cylinder 11. The upper end of the flange 41 provides an anularly disposed shoulder 42 which serves to support the lower end of a piston ring 43. The piston ring 43 may be formed of a suitable yieldable material and in the upper end thereof, there is provided a tapered upwardiy opening recess 44 and the recess 44 serves to receive a tapered retaining nut 47 and it is to be noted that the upper tapered end 46 of the piston ring 43 is disposed outwardly radially to forcibly engage the interior surface 40 of the cylinder 11. The piston retaining nut 47 is threadedly received on the piston rod 30 and engages the piston ring 43 to provide vertical limits of travel of the piston ring 43 on mandrel 30. A suitable set screw 48 may be provided for locking the nut 47 in retaining position. The tapered end of nut 47 serves to urge the portion 45 of the piston ring 43 into engagement with the inner surface of the cylinder 11 to provide centralized engagenent therebetween and the flexible lip 46 of piston ring 43 may be utilized to compensate for wear of the piston ring 43. The nut 47 is provided with slots 45 at the lower end thereof. As best shown in FIGS. 3 and 8, a plurality of circumferentially spaced fluid by-pass grooves 49 are provided in the exterior surface of the piston rod 30 inwardly of the nut 47 and piston ring 43 and the grooves 49 serve to provide communication between portions of the cylinder 11 above the nut 47 and below the piston ring 43. It is to be noted that the grooves 49 terminate at the lower end in passages 50 in the annular flange 41 thereby providing communication between the grooves 49 and the portion of the cylinder below the piston ring 43. It is to be noted that the mandrel 17 is provided with an internal bore 51 of relatively large diameter which is co-extensive with a bore 52 of similar diameter in the piston rod 30 and this permits the passage of a wire line or other tool downwardly through the interior of the tool of this invention without in any way interfering with the operation thereof.

In operation, the tool of this invention is lowered into a well and it is assumed that the lower sleeve 13 will be coupled to a tool or pipe string lodged in the well and which is to be removed therefrom by rotation of the mandrel 17, barrel 10, cylinder 11 and sleeve 13 or in any other desired manner and also, at this time, the piston ring 43 on the piston rod 30 Will occupy the position shown in FIGS. 3 and 5 with the piston ring 43 engaging the portion 40 of reduced diameter in the cylinder 11. The application of an upward force to the mandrel 17 causes movement of such mandrel, as well as the piston rod 30 and piston ring 43 carried thereby upwardly with respect to the cylinder 11 and as the piston ring 43 moves upwardly, fluid within the cylinder 11 above the piston ring 43 Will be compressed to cause a Stretch in the pipe or drill string connected to the mandrel 17 thereby serving to store energy therein. Since the fluid utilized in this tool is largely incompressible, a small portion of the same is permitted to bleed from the portion of the cylinder above the piston ring 43 to the portion of the cylinder below the piston ring 43 through the bypass grooves 49 thereby permitting upward movement of the piston ring 43, but such movement is retarded by the fluid sufliciently to cause Stretch in the drill string as mentioned above. Upon reaching the tapered wall 39 in the cylinder 11 and passing in to the enlarged portion 38, the piston ring 43 will disengage from the inner surface of the cylinder 11, as clearly shown in FIG. 4, thereby providing a relatively large passage between the portion of the cylinder above the piston ring 43 and the portion of the cylinder below the piston ring 43 and this will result in permitting the fluid above the piston ring 43 to flow rapidly downwardly into the lower portion of the cylinder thereby resulting in a sudden rapid, upward movement of the piston rod 30 and mandrel 17 which causes the striking surface 32 on the upper end of the couplng sleeve 25 to engage the downwardly facing impact surface 16 on the lower end of the shoulder 15 to impart a hammer blow thereto and such blow is transmitted through the barrel 10, the cylinder 11 and the sleeve 13 to the tool or other object lodged in the wall to facilitate removal thereof from the well.

Upon completion of the upward stroke of the piston rod 30 and parts associated therewith, if it is desired to repeat the jarring or hammer operation, the piston rod 30 is lowered and since, upon downward movement of the piston ring 43 the same will engage with the tapered wall 39, movement into the portion of reduced diameter 49 is facilitated without undue wear or damage to the piston ring 43 and such ring may move downwardly to the position shown in FIG. 3, whereupon the operation may be repeated. During such downward movement the piston ring 43 moves upwardly away from the shoulder 42 and fluid initially flows from the lower portion of the cylinder to the upper portion between the piston ring 43 and the inner surface of the enlarged portion 38, but after engagement of the piston ring 43 with the portion 40 of reduced diameter, the fluid will flow upwardly through the passages 50 which are enlarged by the upward movement of ring 43 and through by-pass grooves 49.

It will be seen that by the above described invention there has been provided a relatively simple, yet highly eflicient jarring tool in which adjustments may be made to compensate for wear of the piston ring, but in which such wear is reduced to a minimum by reason of the particular Construction of the device and furthermore, such structure serves to eliminate the necessity for the provision of a separate valve means to control flow of fluid between upper and lower portions of the cylinder.

It will be obvious to those skilled in the art that various changes may be made in the invention without departing from the spirit and scope thereof and therefore the invention is not limited by that which is shown in the drawings and described in the specification, but only as indicated in the appended claims.

What is claimed is:

1. A fluid pressure one-way jar comprising an upper elongated tubular member providing a torque barrel, a cylinder removably secured to the lower end of said barrel, an elongated sleeve secured to the lower end of said cylinder, an annular shoulder in the upper end of said barrel providing a downwardly facing impact surface, a hollow torque mandrel slidably received in said barrel, means to prevent rotation of said mandrel with respect to said barrel, means on the upper end of said mandrel for attaching the same to a drill string, a couplng sleeve removably secured to the lower end of said mandrel and providing an enlarged hammer portion having an upwardly facing striking surface, a tubular piston rod removably secured to said couplng sleeve and extending downwardly through said cylinder and into said elongated sleeve, said cylinder having an enlarged internal portion extending from a point adjacent the upper end to a point intermediate the length and terminating in an inwardly tapered wall merging into a portion of reduced diameter extending to the lower end of said cylinder and a piston on said piston rod within said cylinder, said piston comprising an annular flange on said piston rod, a piston ring of yieldable material slidably mounted on said piston rod and supported on the upper surface of said flange, a tapered upwardly opening recess in the upper end of said ring, a nut threadedly received on said piston rod and having a portion for engaging said piston ring to limit sliding movement thereof on said piston rod and fluid by-pass grooves in said piston rod inwardly of said nut and piston ring and providing communication between portions of said cylinder on opposite sides of said piston, whereby upon application of an upward force to said mandrel said piston will move upwardly in said cylinder to compress fluid in said cylinder above said piston ring and accumulate energy in the drill string with a portion of the fluid bleeding by said piston through said grooves and upon said piston ring reaching the enlarged portion in said cylinder pressure in the upper portion of said cylinder will be suddenly released thereby releasing the energy accumulated in the drill string to cause rapid upward movement of said hammer portion and engagement of said striking surface with said impact surface to apply an upward jarring force to said elongated sleeve, said piston partially expanding into the enlarged portion of said cylinder upon said upward movement, said tapered wall serving to contract said piston ring upon subsequent downward movement of said piston with said piston ring moving upwardly into engagement with said nut to permit increased flow of fluid through said grooves to facilitate resetting of the jar for a subsequent operation.

2. A fluid pressure one-way jar comprising an upper elongated tubular member providing a torque barrel, a cylinder removably secured to the lower end of said barrel, an elongated sleeve secured to the lower end of said cylinder, an annular shoulder in the upper end of said barrel providing a downwardly facing impact surface, a hollow torque mandrel slidably received in said barrel, means to prevent rotation of said mandrel with respect to said barrel, means on the upper end of said mandrel for attaching the same to a drill string, a couplng sleeve removably secured to the lower end of said mandrel and providing an enlarged hammer portion having an upwardly facing striking surface, a tubular piston rod removably secured to said couplng sleeve and extending downwardly through said cylinder and into said elongated sleeve, said cylinder having an enlarged internal portion extending from a point adjacent the upper end to a point intermediate the length and terminating in an inwardly tapered wall merging into a portion of reduced diameter extending to the lower end of said cylinder and a piston on said piston rod within said cylinder, said piston comprising an annular flange on said piston rod, a piston ring of yieldable material slidably mounted on said piston rod and supported on the upper surface of said flange, a tapered upwardly opening recess in the upper end of said ring, means to limit sliding movement of said piston ring, and fluid by-pass grooves in said piston rod inwardly of said last-named means, and piston ring and providing communication between portions of said cylinder on opposite sides of said piston, said piston ring being movable between said flange and said last-named means, whereby upon application of an upward force to said mandrel said piston will move upwardly in said cylinder to compress fluid in said cylinder above said piston ring and accumulate energy in the drill string with a portion of the fluid bleeding by said piston through said grooves and upon said piston ring reaching the enlarged portion in said cylinder pressure in the upper portion of said cylinder will be suddenly released thereby releasing the energy accumulated in the drill string to cause rapid upward movement of said hammer portion and engagement of said striking surface with said impact surface to apply an upward jarring force to said elongated sleeve, said piston ring partially expanding into the enlarged portion of said cylinder upon said upward movement, said tapered wall servng to contract said piston ring upon subsequent downward movement of said piston with said piston ring moving upwardly into engagement with said last-named means to permit increased flow of fluid through said grooves to facilitate resetting of the jar for a subsequent operation.

3. A fluid pressure `one-way jar comprising an upper elongated tubular member comprising a torque barrel, a cylinder removably secured to the lower end of said barrel, an elongated sleeve secured to the lower end of said cylinder, an annular shoulder in the upper end of said barrel providing a downwardly facing impact surface, a hollow torque mandrel slidably received in said barrel, means to prevent rotation of said mandrel with respect to said barrel, means on the upper 'end of said mandrel for attaching the same to a drill string, a coupling sleeve removably Secured to the lower end of said mandrel and providing an enlarged hammer portion having an upwardly facing` striking surface, a tubular piston rod removably Secured to said coupling sleeve and extending downwardly through said cylinderand into said elongated sleeve, said cylinder having an enlarged internal portion extending from a point adjacent the upper end to a point intermediate the length and terminating in an inwardly tapered Wall merging into a portion of reduced diameter extending to the lower end of said cylinder and a piston on said piston rod within said cylinder, said piston comprising an annular flange on said piston rod, a piston ring of yieldable material slidably mounted on said piston rod and supported on the upper surface of said flange, a tapered upwardly opening recess in the upper end `of said ring, means on said piston rod for iimiting sliding movement of said piston ring, and fluid by-pass grooves in said piston rod inwardly of said last-named means and piston ring and providing communication between portions of said cylinder on opposite sides of said piston, whereby upon application of an upward force to said mandrel said piston Will move upwardly in said cylinder to compress fluid in said cylinder abov'e said piston ring and accumulate energy in the drill string with a portion of the fluid bleeding by said piston through said grooves and upon said piston ring reaching the enlarged portion in said cylinder pressure in the upper portion of said cylinder will be suddenly re leased thereby releasing the energy accumulated in the drill string to cause rapid upward movement of said hammer portion and engagement of said striking surface with said impact surface to apply an upward jarring force to said elongated sleeve, said piston ring partially expanding into the enlarged portion of said cylinder upon said up- Ward movement, said tapered wall serving to contract said piston ring upon subsequent downward movement of said piston with fluid flowing through said grooves to facilitate resetting of said jar for a subsequent operation.

4. A fluid pressure one-way jar comprising an upper elongated tubular member providing a torque barrel, a cylinder removably secured to the lower end of said barrel, an elongated sleeve secured to the lower end of said cylinder, an annular shoulder in the upper end of said barrel providing a downwardly facing impact surface, a hollow torque mandrel' slidably received in said barrel, means to prevent rotation of said mandrel with respect to said barrel, means on the upper end of said mandrel for attaching the same to a drill string, a coupling sleeve removably Secured to the lower end of said mandrel and providing an enlarged hammer portion having an upwardly facing striking surface, a tubular piston rod renovably Secured to said coupling sleeve and extending downwardly through said cylinder and into said elongated sleeve, said cylinder having an enlarged internal portion extending from a point adjacent the upper end to a point intermediate the length and terminating in an inwardly tapered wall merging into a portion of reduced diameter extending to the lower end of said cylinder and a piston on said piston rod within said cylinder, said piston comprising an annular flange on said piston rod, a piston ring of yieldable material slidably mounted on said piston rod and supported on the upper surface of said flang'e, means on said piston rod to limit sliding movement of said piston ring and fluid by-pass grooves in said piston rod inwardly of said piston ring and providing communication between portions of said cylinder on opposite sides of said piston, whereby upon application of an upward force to said mandrel said piston will move upwardly in said cylinder to compress fluid in said cylinder above said piston ring and accumulate energy in the drill string With a portion of the fluid bieeding by said piston through said grooves and upon said piston ring reaching the enlarged portion in said cylinder pressure in the upper portion of said cylnder Will be suddenly released thereby releasing the energy accumuiated in the drill string to cause rapid Upward movement of said hammer portion and engagement of said striking surface with said impact surface to apply an up- Ward jar'ing force to said elongated sleeve, said piston ring partially expanding into the enlarged portion of said cylinder upon said upward movement, said tapered wall serving to contract said piston ring upon subsequent downward movement of said piston with fluid flowing through said grooves to facilitate resetting of said jar for a subsequent operation.

References Cited by the Examiner UNITED STATES PATENTS 2,981,336 4/1961 Ansite -297 2,987,129 6/1961 Webb 175-297 3,004,616 10/1961 Nutter et al. 175-297 3,005,505 10/1961 Webb 175-297 CHARLES E. OCONNELL, Primary Examz'er. 

4. A FLUID PRESSURE ONE-WAY JAR COMPRISING AN UPPER ELONGATED TUBULAR MEMBER PROVIDING A TORQUE BARREL, A CYLINDER REMOVABLY SECURED TO THE LOWER END OF SAID BARREL, AN ELONGATED SLEEVE SECURED TO THE LOWER END OF SAID CYLINDER, AN ANNULAR SHOULDER IN THE UPPER END OF SAID BARREL PROVIDING A DOWNWARDLY FACING IMPACT SURFACE, A HOLLOW TORQUE MANDREL SLIDABLY RECEIVED IN SAID BARREL, MEANS TO PREVENT ROTATION OF SAID MANDREL WITH RESPECT TO SAID BARREL, MEANS ON THE UPPER END OF SAID MANDREL FOR ATTACHING THE SAME TO A DRILL STRING, A COUPLING SLEEVE REMOVABLY SECURED TO THE LOWER END OF SAID MANDREL AND PROVIDING AN ENLARGED HAMMER PORTION HAVING AN UPWARDLY FACING STRIKING SURFACE, A TUBULAR PISTON ROD REMOVABLY SECURED TO SAID COUPLING SLEEVE AND EXTENDING DOWNWARDLY THROUGH SAID CYLINDER AND INTO SAID ELONGATED SLEEVE, SAID CYLINDER HAVING AN ENLARGED INTERNAL PORTION EXTENDING FROM A POINT ADJACENT THE UPPER END TO A POINT INTERMEDIATE THE LENGTH AND TERMINATING IN AN INWARDLY TAPERED WALL MERGING INTO A PORTION OF REDUCED DIAMETER EXTENDING TO THE LOWER END OF SAID CYLINDER AND A PISTON ON SAID PISTON ROD WITHIN SAID CYLINDER, SAID PISTON COMPRISING AN ANNULAR FLANGE ON SAID PISTON ROD, A PISTON RING OF YIELDABLE MATERIAL SLIDABLY MOUNTED ON SAID PISTON ROD AND SUPPORTED ON THE UPPER SURFACE OF SAID FLANGE, MEANS ON SAID PISTON ROD TO LIMIT SLIDING MOVEMENT OF SAID PISTON RING AND FLUID BY-PASS GROOVES IN SAID PISTON ROD INWARDLY 